1. Field of the Invention
The present invention relates to an over current protection circuit, and more particularly, to an over current protection circuit for a low dropout regulator.
2. Description of the Related Art
A low dropout regulator, one kind of linear regulator, provides an output voltage slightly lower than its input voltage. Like most power supply circuits, a low dropout regulator requires an over current protection mechanism to prevent itself and the load circuit thereof from being damaged by its output current. FIG. 1 shows a conventional low dropout regulator and the over current protection circuit thereof. The low dropout regulator 100, coupled to a load circuit 160, comprises an NMOS power transistor 110, an error amplifier 120 and resistors 130 and 140. The over current protection circuit 150 comprises a current-limiting amplifier 151, a current source 152, resistors 153 and 154 and an NMOS transistor 155.
As shown in FIG. 1, the source electrode of the power transistor 110 is coupled to the load circuit 160. The gate electrode of the power transistor 110 is coupled to the output terminal of the error amplifier 120. One end of the resistor 130 is coupled to the source electrode of the power transistor 110, and the other end of the resistor 130 is coupled to the inverting input terminal of the error amplifier 120. One end of the resistor 140 is coupled to the inverting input terminal of the error amplifier 120, and the other end of the resistor 140 is grounded. The non-inverting input terminal of the error amplifier 120 is coupled to a bandgap voltage. The resistor 153 connects a supply voltage to the drain electrode of the power transistor 110. The resistor 154 connects the supply voltage to the non-inverting input terminal of the current-limiting amplifier 151. The current source 152 is coupled to the non-inverting input terminal of the current-limiting amplifier 151. The gate electrode of the transistor 155 is coupled to the output terminal of the current-limiting amplifier 151. The source electrode of the transistor 155 is grounded. The drain electrode of the transistor 155 is coupled to the output terminal of the error amplifier 120.
The current I1 provided by the current source 152 is fixed, and therefore the voltage across the resistor 154, VA, is also fixed. When the current flowing through the power transistor 110 is over a threshold, i.e., when the voltage across the resistor 153, VB, is higher than the voltage across the resistor 154, VA, the current-limiting amplifier 151 outputs a high voltage to activate the transistor 155. The transistor 155 then pulls down the voltage at the gate electrode of the power transistor 110 to turn off the power transistor 110, and the output current of the low dropout regulator 100 is restrained.
However, since the output current of the power transistor 110 equals the current flowing through the resistor 153, the voltage across the resistor 153, VB, is considerably high. Therefore, the voltage dropout between the supply voltage and the output voltage of the low dropout regulator 100 increases significantly, which contradicts the main function thereof. In addition, the dissipated heat caused by the resistor 153 raises the chip temperature such that the performance of the low dropout regulator 100 is degraded and the heat dissipation problem thereof is aggravated.
In view of the drawbacks of the aforesaid prior art, it is necessary to design a low dropout regulator and an over current protection circuit thereof such that the low dropout regulator is not damaged by an over current, the voltage difference of the input and output voltages of the low dropout regulator does not increase, and the heat dissipation problem is not aggravated.